1. Field of the Invention
The present invention relates to signal processors, filters, and control circuits for controlling output or input of power converter circuits. The present invention also relates to interconnection inverter systems and PWM converter systems using the control circuits.
2. Description of the Related Art
Interconnection inverter systems convert DC (direct current) power which is generated by solar cells for example, to AC (alternate current) power for supply to electrical power systems. (See JP-A-2009-44897, for example.)
FIG. 36 is a block diagram for describing a conventional interconnection inverter system.
The interconnection inverter system A100 converts power generated by a DC power source 1 for supply to a three-phase electrical power system B. Hereinafter, the three phases will be called phase U, phase V and phase W.
An inverter circuit 2 receives a DC voltage from the DC power source 1, and converts the DC voltage into an AC voltage by way of switching operation of switching elements. A filter circuit 3 removes switching frequency components contained in the AC voltage outputted from the inverter circuit 2. A voltage transformer circuit 4 increases (or decreases) the AC voltage from the filter circuit 3 to a system voltage of the electrical power system B. A control circuit 7 receives an electric current signal detected by a current sensor 5 and a voltage signal detected by a voltage sensor 6 as inputs, generates PWM signals based on these, and outputs the PWM signals to the inverter circuit 2. The inverter circuit 2 performs switching operation of the switching elements based on the PWM signals from the control circuit 7.
FIG. 37 is a block diagram for describing an internal configuration of the control circuit 7.
Electrical current signals of respective phases inputted from the current sensor 5 are then inputted to a three-phase to two-phase converter 73.
The three-phase to two-phase converter 73 performs so called three-phase to two-phase conversion (alpha-beta conversion). The three-phase to two-phase conversion is a process in which three-phase AC signals are converted into equivalent two-phase AC signals, by first decomposing the three-phase AC signal components into two kinds of components in an orthogonal coordinate system (hereinafter called fixed coordinate system) composed of a mutually perpendicular two axis called alpha axis and beta axis; and then by adding these components for each of the axes. Thus, the original signal is converted into an AC signal on the alpha axis and an AC signal on the beta axis. Thus, the three-phase to two-phase converter 73 converts the three electric current signals Iu, Iv, Iw inputted thereto, to an alpha axis current signal Iα and a beta axis current signal Iβ, and then outputs these signals to a fixed-to-rotating coordinate converter 78.
The conversion process performed in the three-phase to two-phase converter 73 is represented by a formula shown below as Equation (1).
                              [                                                                      I                  ⁢                                                                          ⁢                  α                                                                                                      I                  ⁢                                                                          ⁢                  β                                                              ]                =                                                                              2                  3                                            ⁡                              [                                                                            1                                                                                      -                                                  1                          2                                                                                                                                    -                                                  1                          2                                                                                                                                                0                                                                                                                3                                                2                                                                                                            -                                                                              3                                                    2                                                                                                                    ]                                      ⁡                          [                                                                    Iu                                                                                        Iv                                                                                        Iw                                                              ]                                ⁢          Λ                                    (        1        )            
The fixed-to-rotating coordinate converter 78 performs so called fixed-to-rotating coordinate conversion (dq conversion). The fixed-to-rotating coordinate conversion converts two-phase signals in the fixed coordinate system into two-phase signals in a rotating coordinate system. The rotating coordinate system is an orthogonal coordinate system composed of d axis and q axis which are perpendicular to each other, and rotating in the same direction and at the same angular velocity as of a fundamental wave of the system voltage in the electrical power system B. The fixed-to-rotating coordinate converter 78 converts alpha axis current signals Iα and beta axis current signals Iβ in the fixed coordinate system which are inputted from the three-phase to two-phase converter 73 into d axis current signals Id and q axis current signals Iq in the rotating coordinate system based on a phase θ of the fundamental wave of the system voltage detected by a phase detector 71, and then outputs the converted signals.
The conversion process performed in the fixed-to-rotating coordinate converter 78 is represented by a formula shown below as Equation (2).
                              [                                                                      I                  ⁢                                                                          ⁢                  d                                                                                                      I                  ⁢                                                                          ⁢                  q                                                              ]                =                                            [                                                                                          cos                      ⁢                                                                                          ⁢                      θ                                                                                                  sin                      ⁢                                                                                          ⁢                      θ                                                                                                                                                          -                        sin                                            ⁢                                                                                          ⁢                      θ                                                                                                  cos                      ⁢                                                                                          ⁢                      θ                                                                                  ]                        ⁡                          [                                                                                          I                      ⁢                                                                                          ⁢                      α                                                                                                                                  I                      ⁢                                                                                          ⁢                      β                                                                                  ]                                ⁢          Λ                                    (        2        )            
An LPF 74a and an LFP 75a are low-pass filters, allowing only DC components in the d axis current signals Id and in the q axis current signals Iq to pass through, respectively. The fixed-to-rotating coordinate conversion converts fundamental wave components in the alpha axis current signal Iα and the beta axis current signal Iβ into DC components of the d axis current signal Id and q axis current signal Iq respectively. A PI controller 74b and a PI controller 75b each perform PI (proportional-integral) control based on the DC component values in the d axis current signals Id and the q axis current signals Iq and deviations from their respective target values, and then output correction value signals Xd, Xq. Since DC components can be used as the target values, the PI controller 74b and the PI controller 75b are capable of providing highly accurate control.
A rotating-to-fixed coordinate converter 79 converts the correction value signals Xd, Xq, which are inputted from the PI controller 74b and the PI controller 75b respectively, into two correction value signals Xα, Xβ in the fixed coordinate system. In other words, it performs a reverse conversion process of the process performed by the fixed-to-rotating coordinate converter 78. The rotating-to-fixed coordinate converter 79 performs so called rotating-to-fixed coordinate conversion (reverse dq conversion), i.e., conversion of the correction value signals Xd, Xq in the rotating coordinate system into correction value signals Xα, Xβin the fixed coordinate system based on the phase θ.
The conversion process performed in the rotating-to-fixed coordinate converter 79 is represented by a formula shown below as Equation (3).
                              [                                                                      X                  ⁢                                                                          ⁢                  α                                                                                                      X                  ⁢                                                                          ⁢                  β                                                              ]                =                                            [                                                                                          cos                      ⁢                                                                                          ⁢                      θ                                                                                                                          -                        sin                                            ⁢                                                                                          ⁢                      θ                                                                                                                                  sin                      ⁢                                                                                          ⁢                      θ                                                                                                  cos                      ⁢                                                                                          ⁢                      θ                                                                                  ]                        ⁡                          [                                                                                          X                      ⁢                                                                                          ⁢                      d                                                                                                                                  X                      ⁢                                                                                          ⁢                      q                                                                                  ]                                ⁢          Λ                                    (        3        )            
A two-phase to three-phase converter 76 converts the correction value signals Xα, Xβ which from the rotating-to-fixed coordinate converter 79 into three correction value signals Xu, Xv, Xw. The two-phase to three-phase converter 76 performs so called two-phase to three-phase conversion process (reverse alpha-beta conversion), i.e., an inverse conversion process of the process performed by the three-phase to two-phase converter 73.
The conversion process performed in the two-phase to three-phase converter 76 is represented by a formula shown below as Equation (4).
                              [                                                                      X                  ⁢                                                                          ⁢                  u                                                                                                      X                  ⁢                                                                          ⁢                  v                                                                                                      X                  ⁢                                                                          ⁢                  w                                                              ]                =                                                                              2                  3                                            ⁡                              [                                                                            1                                                              0                                                                                                                          -                                                  1                          2                                                                                                                                                              3                                                2                                                                                                                                                -                                                  1                          2                                                                                                                                    -                                                                              3                                                    2                                                                                                                    ]                                      ⁡                          [                                                                                          X                      ⁢                                                                                          ⁢                      α                                                                                                                                  X                      ⁢                                                                                          ⁢                      β                                                                                  ]                                ⁢          Λ                                    (        4        )            
A PWM signal generator 77 generates PWM signals based on the correction value signals Xu, Xv, Xw from the two-phase to three-phase converter 76, and outputs the generated signals.
A problem, however, is that designing the control system of the control circuit 7 requires tremendous work. Recent interconnection inverter systems must satisfy requirements for very quick response in its control operation, such as restoring its output within a predetermined amount of time upon momentary voltage drop. In order to design the control system so as to satisfy such requirements as the above, the LPF 74a and the LFP 75a must be given optimal parameters, and the PI controller 74b and the PI controller 75b must be designed to have optimized proportional and integral gains. However, since the fixed-to-rotating coordinate converter 78 and the rotating-to-fixed coordinate converter 79 perform nonlinear time-varying processes, it was not possible to design the control system by using a linear control theory. Further, system analysis was not possible, either, since the control system includes nonlinear time-varying processing.